I have a 1996 Jaguar XJ6 X300 fitted with a Stefanelli LPG system and recently all 4 of the green LEDs on the dash have been lit irrespective of how much fuel there is in the tank. Occasionally the right hand LED goes out, but in normal service it's the left hand one that goes out first as the level drops.

I imagine the sender unit is nothing like a conventional one for a petrol tank?

The sender for LPG is exactly like a conventional one for a petrol tank It's a float on an arm connected to a variable resistor. Maybe check the wiring to it isn't unplugged or damaged.

As there are various different types of tank and sensor, the software for your LPG system will have a section where you select the type of sensor (ie whether the voltage goes up or down as the tank empties) and the voltages that represent different levels on the gauge. It might be worth checking that they are set correctly if you have the cable and software.

On some systems there's a procedure that will auto-calibrate the gauge without the software when you're at the filling station. For example on my KME Nevo you do the following with an empty tank so running on petrol: Ignition on, hold switch for 15 seconds until LEDs flash, ignition off. Fill the tank to the top (pay for fuel). Ignition on, wait for the LEDs to stop flashing, start engine and drive off.

It's a float arm but not directly connected to the gauge and sender, the arm is inside the tank and the sender unit it outside and they are magnetically coupled. Have a look at the gauge on the tank, if the reading corresponds with what is in the tank, it is likely an electrical problem. If the gauge is reading wrong either the gauge or the float arm are stuck.

If you can access the gauge its possible to change the sender without needing to release the contents of the tank. Its also possible to use a screwdriver on the back of it to get it to move around the reading, you should see this changing the reading on the gauge. Assuming that theres no damage to the wiring, its likely the sender has given up

I traced the white and black wires from the sender unit to the front of the car and found a bodged repair that someone had done inside the rear wheel arch using a "chocker block"

I removed this and fitted 3.9mm motorcycle type bullet connectors, which are a less bodgy bodge in my view. This took a good hour and a half as I had to extend one wire by soldering on an extra piece after accidentally cutting it too short - not happy.

I was even less happy after re-fitting the wheel and taking it for a drive only to find all 4 lights still on........I didn't have the energy to see if something in my repair had parted company, so will leave that for another day.